Question

Carbohydrates are optically active polyhydroxy aldehydes or ketones or the compounds which produce such units on hydrolysis. They have been broadly classified into three groups - monosaccharides, oligosaccharides and polysaccharides. The carbohydrates may also be classified as either reducing or non-reducing sugars. An important monosaccharide glucose is an aldohexose and is correctly named as D(+)-glucose. It was assigned the open structure on the basis of its reactions with HI, \(\mathrm{NH_2OH}\), \(\mathrm{Br_2}\) water, \((\mathrm{CH_3CO})_2\mathrm{O}\) and nitric acid.

Despite having the \(-\mathrm{CHO}\) group, glucose does not give Schiff's test and hydrogen sulphite addition product with \(\mathrm{NaHSO_3}\). It was found that glucose forms a six-membered ring in which one of the \(-\mathrm{OH}\) groups add to the \(-\mathrm{CHO}\) group and form a cyclic hemiacetal structure.

In D(+)-glucose, what do the letter 'D' and sign '(+)' represent ?

Hint:

D is the configuration of the last chiral carbon (-OH on the right), and (+) means dextrorotatory.

Answer 1

Step 1: Understanding the Concept:
The name D(+)-glucose has two separate labels. One label tells us about the arrangement of atoms in space. The other tells us how the sugar turns plane polarised light.

Step 2: Meaning of the letter 'D'.
Draw glucose as a Fischer projection with the -CHO group on top. Look at the chiral carbon farthest from the -CHO group, which is C-5.
In glucose the -OH group on C-5 points to the right. This is the same arrangement as in D-(+)-glyceraldehyde.
So 'D' tells the relative configuration of C-5. It has nothing to do with the direction of rotation.

Step 3: Meaning of the sign '(+)'.
The sign '(+)' means dextrorotatory. A solution of glucose rotates the plane of polarised light to the right (clockwise).
The two labels are independent. For example, D-fructose is D but levorotatory, so it is written D(-).

Final Answer:
'D' shows that the -OH group on the last chiral carbon (C-5) is on the right, as in D-glyceraldehyde. '(+)' shows that glucose is dextrorotatory.